Reports: AC10
45023-AC10 Laser- and Electric-Field-Induced Nucleation in Supersaturated Solutions and Supercooled Melts
Nonphotochemical laser-induced nucleation (NPLIN), a phenomenon in which intense laser pulses induce supersaturated solutions to nucleate, was discovered in our laboratory about a dozen years ago. In addition to providing control of the time and place of nucleation, this phenomenon provides control of polymorphism in some systems, through the polarization of the laser light, an effect we call “polarization switching”.
A highlight of our research supported by this grant was the discovery of a second system that exhibits “polarization switching”: aqueous L-histidine. Supersaturated aqueous L-histidine solutions were exposed to nanosecond green laser pulses. Circularly polarized laser pulses tended to nucleate the orthorhombic A polymorph, whereas linearly polarized pulses tended to nucleate a mixture of the orthorhombic A and monoclinic B polymorphs.
We also found that NPLIN could be observed in a supercooled nematic liquid crystal, 5CB. Laser exposure during cooling results in fewer defects than are found without laser exposure, and the nematic director tends to be aligned along the direction of the plane of polarization of the light.
We have continued to explore the range of systems that exhibit NPLIN. Two undergraduate students, Radu Iliescu and Quinn Gibson have studied NPLIN in a supercooled melt of salol (phenyl salicylate). They have succeeded in making metastable samples of salol supercooled to room temperature, and will shortly be exposing such solutions to green laser pulses. Another undergraduate student, David Miret, studied NPLIN in supersaturated solutions of the pharmaceutical drug carbamazepine dissolved in methanol.
We have also begun to study both spontaneous and laser-induced nucleation in levitated supersaturated microdroplets. By studying nucleation in the small volumes of microdroplets, we can achieve considerably higher supersaturations than in bulk solutions. Graduate student, Ke Fang, who was supported by this grant, succeeded in generating and levitating micron-sized solution droplets in a dynamic levitator trap. By lowering and raising the solvent vapor pressure, Fang was able to nucleate and resolvate these droplets, respectively.
